It is well known that an aromatic polyimide asymmetric composite membrane or film can be produced from a dope solution of a corresponding polyamic acid, which is a precursor polymer of the polyimide, by means of a wet membrane-forming process. For example, the above-mentioned type of process is disclosed in Japanese patent application Laid-open (Kokai) No. 49-45152. In this type of process, the dope solution containing the polyamic acid is formed into a thin film and the thin dope solution film is coagulated by immersing it in a coagulating liquid containing an imide-cyclization compound consisting essentially of a lower aliphatic carboxylic acid and a tertiary amine. In this coagulating procedure, the polyamic acid is imide-cyclized so as to be converted into the corresponding polyimide. That is, in this conventional coagulating procedure, the coagulation of the thin dope solution film is carried out simultaneously with the imide-cyclization of the aromatic polyamic acid. Therefore, it is difficult to stably produce the polyimide separating membrane having a satisfactory gas separating property in a satisfactory reproductivity thereof. Also, the resultant product is an asymmetrical composite membrane and, therefore, exhibits a relatively poor permeating rate of the gases when used as a gas separating or gas concentrating membrane.
Under the above-mentioned circumstances, it was strongly desired to provide a new process for producing an aromatic polyimide membrane having a porous structure and exhibiting an enhanced permeating rate of a gas or liquid therethrough and, therefore, being useful as a separator of a gas or liquid mixture and as a concentrator of a gas or liquid, from a polyamic acid.
The inventors of the present invention found that in the wet membrane-forming process in which a dope solution containing an aromatic polyamic acid is used, an aromatic polyimide separating membrane which is free from the above-mentioned disadvantages of the conventional separating membranes, could be produced by forming the thin dope solution film at a relatively low temperature; by coagulating the dope solution film in a coagulating liquid containing, as a main component, water and/or at least one lower aliphatic alcohol so as to form a precursory polyamic acid membrane; and by drying and then heating the coagulated polyamic acid membrane at an elevated temperature so as to imide-cyclize the amide-acid structure in the polyamic acid into an imide structure and to provide an aromatic polyimide separating membrane.